Method and system for rebalancing electrolytes in a redox flow battery system

ABSTRACT

A method of rebalancing electrolytes in a redox flow battery system comprises directing hydrogen gas generated on the negative side of the redox flow battery system to a catalyst surface, and fluidly contacting the hydrogen gas with an electrolyte comprising a metal ion at the catalyst surface, wherein the metal ion is chemically reduced by the hydrogen gas at the catalyst surface, and a state of charge of the electrolyte and pH of the electrolyte remain substantially balanced.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/832,671, filed Jun. 7, 2013 and entitled METHOD AND SYSTEMFOR REBALANCING ELECTROLYTES IN A REDOX FLOW BATTERY SYSTEM, the entiredisclosure of which is herein incorporated by reference.

BACKGROUND AND SUMMARY

Redox flow batteries store electrical energy in a chemical form andsubsequently dispense the stored energy in an electrical form via aspontaneous reverse redox reaction. Conversion between the chemical andelectrical energy occurs in a reactor cell. One issue with conventionalredox flow batteries is that over time the electrolyte state of chargecan become imbalanced, thereby decreasing battery capacity due tohydrogen generation from the electrolyte via side reactions. Forexample, hydrogen gas is emitted as an electrochemical byproduct duringbattery charging. As another example, in a hybrid redox flow battery,hydrogen gas is emitted as a byproduct of a corrosion reaction at anegative (plating) electrode. Because hydrogen gas production consumesprotons instead of the electro-active material in the battery, hydrogengas generation not only results in an electrolyte state of chargeimbalance which reduces the battery capacity, but also a rise inelectrolyte pH which can lead to electrolyte stability issues.

Electrolyte rebalancing methods and systems typically employ anauxiliary rebalancing cell (electrochemical or photochemical) to convertthe hydrogen gas back to protons via an auxiliary electrochemicalreaction. For example, Thaller (U.S. Pat. No. 4,159,366) discloses aredox flow system including an electrochemical rebalancing cell, wherehydrogen gas evolved from the battery negative electrode flows throughthe rebalancing cell anode and positive electrolyte flows through therebalancing cell cathode. Electrochemical reactions occurring at theelectrodes of the rebalancing cell convert gaseous hydrogen back toprotons, consume the imbalanced positive electrolyte, and rebalance theelectrochemical capacity of the positive and negative electrolytes.

The inventors have recognized various issues with the above system.Namely, electrochemical fuel cells are complex systems that are costlyto manufacture and to operate. A simpler and cheaper effectivealternative to providing an auxiliary cell for rebalancing electrolytesin redox flow batteries is needed.

One approach that addresses the above issues is a method of rebalancingelectrolytes in a redox flow battery system, comprising directinghydrogen gas generated on the negative electrode of the redox flowbattery system to a catalyst surface, and fluidly contacting thehydrogen gas with the positive electrolyte comprising a metal ion at thecatalyst surface. Such operation may occur with the positive metal ionchemically reduced by the hydrogen gas at the catalyst surface and/or astate of charge of the electrolyte substantially balanced. In oneembodiment, the metal ion may comprise an imbalanced positive metal ion.In another embodiment, the electrolyte pH and the electrolyte state ofcharge remain substantially balanced.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 0 is a schematic of an example of a redox flow battery.

FIG. 1 is an example Pourbaix diagram for iron.

FIG. 2 is a schematic of an example process for testing a rebalancingreaction for a redox flow battery system.

FIG. 3 is a graph showing an example cyclic voltammogram during arebalancing reaction test for a redox flow battery system.

FIG. 4 is a graph showing an example solution metal ion and protonconcentration profile during a rebalancing reaction test for a redoxflow battery system.

FIG. 5 is a graph showing an example rebalancing reaction ratedependence on hydrogen concentration.

FIG. 6 is a graph showing an example rebalancing reaction Arrheniusplot.

FIG. 7 is a schematic showing an example apparatus for rebalancing aredox flow battery system.

FIGS. 8-10 are schematics showing a cross-section of an example redoxflow battery.

FIG. 11 is a flow chart for an example method of rebalancingelectrolytes in a redox flow battery system.

DETAILED DESCRIPTION

The present description relates to methods and systems for rebalancingelectrolytes in a redox flow battery system. The description primarilydescribes an all-iron hybrid redox flow battery (IFB) as an exampleredox flow battery system, however the methods and systems forrebalancing electrolytes disclosed in the present description also applyto other types of redox flow batteries such as an iron/chromium redoxflow battery system.

FIG. 0 illustrates an example schematic for a redox flow battery. FIG. 1illustrates a Pourbaix diagram for iron, showing the impact of changingelectrolyte pH on electrolyte stability. FIG. 2 illustrates an exampleprocess for testing a rebalancing reaction for rebalancing electrolytesin a redox flow battery system. FIGS. 3-6 show example graphs of testdata used for characterizing the rebalancing reaction for rebalancingelectrolytes in a redox flow battery system. FIGS. 7-10 illustratesexample embodiments of systems and methods for rebalancing electrolytesin redox flow battery systems, and FIG. 11 is a flowchart for an examplemethod of rebalancing electrolytes in redox flow battery systems.

Referring now to FIG. 0, operation of a redox flow battery system 10 isdescribed. The reduction-oxidation (redox) flow battery is anelectrochemical storage device that stores energy in a chemical form andconverts the stored chemical energy to an electrical form viaspontaneous reversible redox reactions. The reaction in a flow batteryis reversible, so conversely, the dispensed chemical energy can berestored by the application of an electrical current inducing thereverse redox reactions. A single redox flow battery cell 18 generallyincludes a negative electrode compartment 20, a separator 24, and apositive electrode compartment 22. The negative electrode compartment 20may comprise a negative electrode 26, and a negative electrolytecomprising electro-active materials. The positive electrode compartment22 may comprise a positive electrode 28, and a positive electrolytecomprising electro-active materials. In some examples, multiple cells 18may be combined in series or parallel to create a higher voltage orcurrent in a redox flow battery system. Electrolytes are typicallystored in tanks external to the cell, and are pumped via pumps 30 and 32through the negative electrode compartment 20 side and the positiveelectrode compartment 22 side of the battery, respectively. In theexample of FIG. 0, the positive electrolyte is stored at a positiveelectrolyte source 52, which may comprise an external positiveelectrolyte tank, and the negative electrolyte is stored at a negativeelectrolyte source 50, which may comprise a second external tank. Theseparator 24 may comprise an electrically insulating ionic conductingbarrier which prevents bulk mixing of the positive electrolyte and thenegative electrolyte while allowing conductance of specific ionstherethrough. For example, the separator 24 may comprise an ion-exchangemembrane or a microporous membrane.

When a charge current is applied to the battery terminals 40 and 42, thepositive electrolyte is oxidized (lose one or more electrons) at thepositive electrode 28, and the negative electrolyte is reduced (gain oneor more electrons) at the negative electrode 26. During batterydischarge, reverse redox reactions occur on the electrodes. In otherwords, the positive electrolyte is reduced (gain one or more electrons)at the positive electrode 28, and the negative electrolyte is oxidized(lose one or more electrons) at the negative electrode 26. Theelectrical potential difference across the battery is maintained by theelectrochemical redox reactions in the positive electrode compartment 22and the negative electrode compartment 20, and can induce a currentthrough a conductor while the reactions are sustained. The amount ofenergy stored by a redox battery is limited by the amount ofelectro-active material available in electrolytes for discharge,depending on the total volume of electrolytes and the solubility of theelectro-active materials.

During operation of a redox flow battery system, sensors and probes maymonitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, sensors 62 and 60 maybe be positioned to monitor positiveelectrolyte and negative electrolyte conditions at the positiveelectrolyte source 52 and the negative electrolyte source 50,respectively. As another example, sensors 72 and 70 may monitor positiveelectrolyte and negative electrolyte conditions at the positiveelectrode compartment 22 and the negative electrode compartment 20,respectively. Sensors may be positioned at other locations throughoutthe redox flow battery system to monitor electrolyte chemical propertiesand other properties. For example a sensor may be positioned in anexternal acid tank (not shown) to monitor acid volume or pH of theexternal acid tank, wherein acid from the external acid tank is suppliedvia an external pump (not shown) to the redox flow battery system inorder to reduce precipitate formation in the electrolytes. Additionalexternal tanks and sensors may be installed for supplying otheradditives to the redox flow battery system. Sensor information may betransmitted to a controller 80 which may in turn actuate pumps 30 and 32to control electrolyte flow through the cell 18, or to perform othercontrol functions, as an example. In this manner, the controller 80 maybe responsive to, one or a combination of sensors and probes.

Hybrid flow batteries are redox flow batteries that are characterized bythe deposit of one or more of the electro-active materials as a solidlayer on an electrode. Hybrid batteries may, for instance, include achemical that plates via an electrochemical reaction as a solid on asubstrate throughout the battery charge process. During batterydischarge, the plated species may ionize via an electrochemicalreaction, becoming soluble in the electrolyte. In hybrid batterysystems, the charge capacity (e.g., amount of energy stored) of theredox battery may be limited by the amount of metal plated duringbattery charge and may accordingly depend on the efficiency of theplating system as well as the available volume and surface areaavailable for plating.

In a hybrid flow battery system the negative electrode 26 may bereferred to as the plating electrode and the positive electrode 28 maybe referred to as the redox electrode. The negative electrolyte withinthe plating side (e.g., negative electrode compartment 20) of thebattery may be referred to as the plating electrolyte and the positiveelectrolyte on the redox side (e.g. positive electrode compartment 22)of the battery may be referred to as the redox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Accordingly,during charge, the negative electrolyte and negative electrode may berespectively referred to as the catholyte and cathode of theelectrochemical reaction, while the positive electrolyte and thepositive electrode may be respectively referred to as the anolyte andanode of the electrochemical reaction. Alternatively, during discharge,the negative electrolyte and negative electrode may be respectivelyreferred to as the anolyte and anode of the electrochemical reaction,while the positive electrolyte and the positive electrode may berespectively referred to as the catholyte and cathode of theelectrochemical reaction.

For simplicity, the terms positive and negative are used herein to referto the electrodes, electrolytes, and electrode compartments in redoxbattery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode,ferrous ion, Fe²⁺, receives two electrons and plates as iron metal on tothe negative electrode 26 during battery charge, and iron metal, Fe⁰,loses two electrons and re-dissolves as Fe²⁺ during battery discharge.At the positive electrode, Fe²⁺ loses an electron to form ferric ion,Fe³⁺, during charge, and during discharge Fe³⁺ gains an electron to formFe²⁺: The electrochemical reaction is summarized in equations (1) and(2), wherein the forward reactions (left to right) indicateelectrochemical reactions during battery charge, while the reversereactions (right to left) indicate electrochemical reactions duringbattery discharge:

Fe²⁺+2e ⁻

Fe⁰−0.44 V(Negative Electrode)  (1)

2Fe²⁺

2Fe³⁺+2e ⁻+0.77 V(Positive Electrode)  (2)

As discussed above, the negative electrolyte used in the all iron redoxflow battery (IFB) may provide a sufficient amount of Fe²⁺ so that,during charge, Fe²⁺ can accept two electrons from the negative electrodeto form Fe⁰ and plate onto a substrate. During discharge, the plated Fe⁰may then lose two electrons, ionizing into Fe²⁺ and be dissolved backinto the electrolyte. The equilibrium potential of the above reaction is−0.44V and thus this reaction provides a negative terminal for thedesired system. On the positive side of the IFB, the electrolyte mayprovide Fe²⁺ during charge which loses electron and oxidizes to Fe³⁺.During discharge, Fe³⁺ provided by the electrolyte becomes Fe²⁺ byabsorbing an electron provided by the electrode. The equilibriumpotential of this reaction is +0.77V, creating a positive terminal forthe desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode may be coupled via terminal40 to the negative side of a voltage source so that electrons may bedelivered to the negative electrolyte via the positive electrode (e.g.,as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in the positiveelectrode compartment 22). The electrons provided to the negativeelectrode 26 (e.g., plating electrode) can reduce the Fe²⁺ in thenegative electrolyte to form Fe⁰ at the plating substrate causing it toplate onto the negative electrode.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofcell 18 to provide additional Fe³⁺ ions via an external source, such asan external positive electrolyte tank 52. More commonly, availability ofFe⁰ during discharge may be an issue in IFB systems, wherein the Fe⁰available for discharge may be proportional to the surface area andvolume of the negative electrode substrate as well as the platingefficiency. Charge capacity may be dependent on the availability of Fe²⁺in the negative electrode compartment 20. As an example, Fe²⁺availability can be maintained by providing additional Fe²⁺ ions via anexternal source, such as an external negative electrolyte tank 50 toincrease the concentration or the volume of the negative electrolyte tothe negative electrode compartment 20 side of cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough the separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) can result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ can damage the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, an adding to process cost and complexity.Adding specific organic acids to the positive electrolyte and thenegative electrolyte in response to electrolyte pH changes may alsomitigate precipitate formation during battery charge and dischargecycling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries.

Further still, iron redox flow batteries reduce the use of toxic rawmaterials and can operate at a relatively neutral pH as compared toother redox flow battery electrolytes. Accordingly, IFB systems reduceenvironmental hazards as compared with all other current advanced redoxflow battery systems in production.

Turning now to FIG. 1, it illustrates an example of a Pourbaix diagram.A Pourbaix diagram maps out possible stable equilibrium phases of anaqueous electrochemical system. The various solid lines in the Pourbaixdiagram of FIG. 1 represent equilibrium conditions where the indicatedspecies on either side of the line have the same chemical activity.Inside the regions on either side of the solid lines, the correspondingspecies predominates. In this way, Pourbaix diagrams can illustrate howpH changes can affect electrolyte species and stability in a redox flowbattery system such as an IFB, leading to cycling performance lossesover time. As an example, FIG. 1 shows a Pourbaix diagram 100 for iron.The vertical axis of FIG. 1 represents the potential with respect to thestandard hydrogen electrode, while pH is represented on the horizontalaxis. During charge of an IFB, for example, ferrous ion, Fe²⁺, isreduced (accepts two electrons in a redox reaction) to metal iron, Fe⁰,at the negative electrode. Simultaneously, at the positive electrode,ferrous ion, Fe²⁺, is oxidized (loss of an electron) to ferric ion,Fe³⁺. Concurrently, at the negative electrode, the ferrous ironreduction reaction competes with the reduction of protons, H⁺, whereintwo protons each accept a single electron to form hydrogen gas, H₂ andthe corrosion of iron metal to produce ferrous ion, Fe²⁺. The productionof hydrogen gas through reduction of hydrogen protons and the corrosionof iron metal are shown in equations (3) and (4), respectively:

H⁺ +e

½H₂(proton reduction)  (3)

Fe⁰+2H⁺

Fe²⁺+H₂(iron corrosion)  (4)

As a result, the negative electrolyte in the negative electrodecompartment 20 tends to stabilize at a pH range between 3 and 6, whereinformation of ferrous hydroxide ion (FeOH⁺) at 112, precipitation offerric hydroxide, Fe(OH)₃ at 114, and hydrogen evolution at 104 are allreduced. At the positive electrode compartment 22, ferric ion, Fe³⁺, hasa much lower acid disassociation constant (pKa) than that of ferrousion, Fe²⁺. Therefore, as more ferrous ions are oxidized to ferric ions,the positive electrolyte tends to stabilize at a pH less than 2, inparticular at a pH closer to 1 within region 120.

Accordingly, maintaining the positive electrolyte pH in a first range inwhich the positive electrolyte (positive electrode compartment 22)remains stable and maintaining the negative electrolyte pH in a secondrange in which the negative electrolyte (negative electrode compartment20) remains stable may reduce low cycling performance and increaseefficiency of redox flow batteries. For example, maintaining a pH of anegative electrolyte in an IFB between 3 and 4 may reduce iron corrosionreactions and increase iron plating efficiency, while maintaining a pHof a positive electrolyte less than 2, in particular less than 1, maypromote the ferric/ferrous ion redox reaction and reduce ferrichydroxide formation.

As indicated by equation (3) and (4), evolution of hydrogen can causeelectrolyte imbalance in a redox flow battery system. For example,during charge, electrons flowing from the positive electrode to thenegative electrode (e.g., as a result of ferrous ion oxidation), may beconsumed by hydrogen evolution via equation (3), thereby reducing theelectrons available for plating given by equation (1). Because of thereduced plating, battery charge capacity is reduced. Additionally,corrosion of the iron metal further reduces battery capacity since adecreased amount of iron metal is available for battery discharge. Thus,an imbalanced electrolyte state of charge between the positive electrodecompartment 22 and the negative electrode compartment 20 can develop asa result of hydrogen production via reaction (3) and (4). Furthermore,hydrogen gas production resulting from iron metal corrosion and protonreduction both consume protons, which can result in a pH increase of thenegative electrolyte. As discussed above with reference to FIG. 1, anincrease in pH may destabilize the electrolyte in the redox batter flowsystem, resulting in further battery capacity and efficiency losses.

An approach that addresses the electrolyte rebalancing issues that canbe caused by hydrogen gas production in redox flow battery systemscomprises reducing the imbalanced ion in the positive electrolyte withhydrogen generated from the side reactions. As an example, in an IFBsystem, the positive electrolyte comprising ferric ion may be reduced bythe hydrogen gas according to equation (5):

Fe³⁺+½H₂→Fe²⁺+H⁺  (5)

In the IFB system example, by reacting ferric ion with hydrogen gas, thehydrogen gas can be converted back to protons, thereby maintain asubstantially constant pH in the negative electrode compartment 20 andthe positive electrode compartment 22. Furthermore, by converting ferricion to ferrous ion, the state of charge of the positive electrolyte inthe positive electrode compartment 22 may be rebalanced with the stateof charge of the negative electrolyte in the negative electrodecompartment 20. Although equation (5) is written for rebalancingelectrolytes in an IFB system, the method of reducing an electrolytewith hydrogen gas may be generalized by equation (6):

$\begin{matrix}\left. {M^{x +} + {\frac{\left( {x - z} \right)}{2}H_{2}}}\rightarrow{M^{z +} + {\left( {x - z} \right)H^{+}}} \right. & (6)\end{matrix}$

In equation (6), M^(x+) represents the positive electrolyte M havingionic charge, x, M^(z+) represents the reduced electrolyte M havingionic charge, z.

A catalyst comprising graphite or comprising supported precious metal(e.g., carbon-supported Pt, Rd, Ru, or alloys thereof) catalyst mayincrease the rate of reaction described by equation (5) for practicalutilization in a redox flow battery system. As an example, hydrogen gasgenerated in the redox flow battery system may be directed to a catalystsurface, and hydrogen gas and electrolyte (e.g., comprising ferric ion)may be fluidly contacted at the catalyst surface, wherein the hydrogengas chemically reduces the ferric ion to ferrous ion and producespositive hydrogen ions (e.g., protons). As described above, the catalystsurface may comprise graphite. In some examples, the reaction describedby equation (5) may proceed at a faster rate when the catalyst comprisesa precious metal-based catalyst, such as carbon-supported Pt, Rd, Ru, oralloys thereof. As an example, in cases where the partial pressure ofhydrogen gas (e.g. hydrogen gas concentration) is high and when a slowerrate of reaction can be tolerated, the less costly graphite catalyst maybe used. On the other hand, a small amount (e.g., 0.2 to >0.5 wt %) ofprecious metal catalyst supported on carbon can increase the rate ofreaction as compared to using a graphite catalyst. Different types ofcatalysts, such as Pt, Pd, Ru or alloys of the above, and the like, anddifferent amounts (0.2 to >0.5 wt %) thereof can be utilized dependingon a reaction speed for any specific battery system. Furthermore, alloysof the catalyst can be utilized to reduce cost and increase corrosionstability of the catalyst. For example 10% addition of rhodium toplatinum can reduce the corrosion of platinum by the ferric ion by over98% (Handbook of Corrosion Data, Bruce D. Craig, David S. Anderson).

Turning now to FIG. 2, it illustrates a schematic of an example processfor testing a rebalancing reaction for a redox flow battery system, forexample, an IFB, according to equation (5). As shown in FIG. 2,electrolyte solution 242 comprising a metal ion (e.g., ferric ion) ofknown concentration may be pumped from an electrolyte source 240 (e.g.,a storage tank) at a measured flow rate to a trickle bed reactor 230. Ahydrogen gas source 220 (e.g., a hydrogen gas cylinder) may supplyhydrogen gas to the trickle bed reactor 230 at a flow rate regulated bya metering device 222, for example, a rotameter. In addition an inertdiluent, for example argon gas, may be supplied from a diluent source210 to the trickle bed reactor 230 at a flow rate regulated by ametering device 212, for example, a rotameter. The electrolyte solutioncomprising the metal ion (e.g. ferric ion solution) may be mixed withthe entering hydrogen and diluent gases at the inlet of the trickle bedreactor 230, thereby flowing through the trickle bed reactor 230 as agas-liquid mixture. The trickle bed reactor 230 comprises a packedcatalyst bed 234, the packed catalyst bed 234 comprising closely-packedcatalyst particles. The closely-packed catalyst particles may compriseinterparticle and intraparticle pores through which fluid (e.g., liquid,gas, or a mixture thereof) can flow and in which fluids can fluidlycontact the surfaces of the catalyst particles. For example, thecatalyst bed 234 may comprise carbon-supported precious metal catalystparticles and/or graphite catalyst particles. As the gas-liquid mixtureof hydrogen, diluent, and electrolyte flow downwards over the catalystbed 234, the hydrogen gas and the liquid electrolyte are fluidlycontacted over the surfaces of the catalyst bed 234. Because thecatalyst bed 234 comprises packed catalyst particles with interparticleand intraparticle pores, the catalyst surface area per volume ofcatalyst exposed for fluidly contacting the hydrogen gas and the liquidelectrolyte can be increased, thereby facilitating the reactiontherebetween. Furthermore, because the gas-liquid mixture of hydrogengas and liquid electrolyte trickles through the catalyst bed 234, acontact time for fluidly contacting the hydrogen gas and the liquidelectrolyte can be increased, thereby facilitating the reduction of themetal ion by the hydrogen gas at the catalyst surface according toequations (5) or (6).

The reaction products of the trickle bed reactor 230 are returned to theelectrolyte source 240. A sensor 270 may comprise one or a plurality ofsensors and/or measurement devices to measure chemical properties ofelectrolyte solution 242. As an example, sensor 270 may comprise apotentiostat and a three-electrode sensor comprising a glassy carbonworking electrode, a platinum mesh counter electrode, and an Ag/AgClreference electrode for performing cyclic voltammetry to measure dynamicelectrolyte concentration profiles. The potentiostat may be set at ascan rate of 1/minute to analyze the electrolyte solution 242. For thecase of an IFB system, the electrolyte solution comprises ferric ionsolution, and the cyclic voltammetry may determine the changes in ferricion concentration in the electrolyte solution by measuring theferric-to-ferrous reduction peak. Sensor 270 may further comprise a pHmeter for measuring changes in the ferric ion solution pH during thetest.

In order to characterize the reaction kinetics of the ferric ionreduction reaction, a model for the trickle bed reactor 230 may berepresented by equations (7)-(9):

$\begin{matrix}{{- r^{\prime}} = {N\frac{X}{W}}} & (7) \\{r^{\prime} = {r*\rho_{cat}}} & (8) \\{R = {{kC}_{{Fe}\; 3}^{x}*C_{H\; 2}^{y}}} & (9)\end{matrix}$

where r′=rate of reaction (mol/s-g), r=rate of reaction (mol/s-l),N=molar flow (mol/s), dX=conversion (%), dW=weight of catalyst (g),ρ_(cat)=density of catalyst (g/l), C=concentration of species,k=reaction constant (l/s), x=the order of reaction on ferric ionconcentration, and y=the order of reaction on hydrogen partial pressure.

An example of the test results is shown in FIGS. 3-4, where a ferricsolution starting concentration was 0.1 mol/l and hydrogen partialpressure was 100%. Ferric solution flow rate was set to 0.00067 l/s.FIG. 3 illustrates a graph 300 plotting the measured current vs. themeasured cell potential relative to the Ag/AgCl reference electrode.Graph 300 shows how the ferric to ferrous reduction peak 320 reduces asthe reaction proceeds with time (indicated by arrow 350), suggesting adecreased ferric ion concentration with time. Since the peak height inthe cyclic voltammogram is directly proportional to concentration, theferric ion concentration change may be calculated from the ratio ofcyclic voltammogram reduction peak change as the reaction proceeds.Furthermore, as ferric ion is reacted and as ferric ion concentrationdecreases as per equation (5), proton concentration concomitantlyincreases, causing a decrease in solution pH. The solution pH changewith time may be monitored with a pH meter. FIG. 4 shows a graph 400 ofthe example test ferric ion concentration 410 and solution pH 420profiles decreasing with time.

Using the above-described apparatus, a series of experiments may becarried out to characterize the reaction kinetics, in particular, tostudy how the reaction rate changes with ferric ion concentration andhydrogen partial pressure. In experiments carried out by the subjectinventors, it was found that varying ferric ion concentration did notinfluence the reaction rate. Thus, it was determined that the order ofthe reaction (equation (5)) with respect to ferric ion concentration waszero (e.g., x=0 in equation (9)).

Next, FIG. 5 illustrates a graph 500 showing the reaction ratedependence on hydrogen partial pressure. In the test experiments fordetermining the order of the reaction with respect to hydrogen partialpressure, the hydrogen gas partial pressure was varied by adjusting theflow rate of inert argon diluent gas. The dashed line 510 is a fittedtrend line based on the experimental data (solid diamonds) and may aidin determining the order of the reaction with respect to hydrogenpartial pressure (e.g., y in equation (9). The order of the reactionwith respect to hydrogen partial pressure may aid in battery operationbecause hydrogen concentration may decrease with time as it reacts withferric ions.

Next, FIG. 6 illustrates an Arrhenius plot 600 showing how the ferricreduction reaction by hydrogen gas is affected by reaction temperature.Experimental data shown in FIG. 6 may be generated from utilizing theexperimental test setup of FIG. 2 by varying the ferric ionconcentration and hydrogen partial pressure, and varying the reactiontemperature. The reaction temperature may be varied by heating orcooling the trickle bed reactor 230 and the catalyst bed 234, theelectrolyte source 240, and the hydrogen gas source 220, or acombination thereof, as an example. Using equations (7)-(9), thereaction rate constant, k, may be calculated at each reaction condition(e.g., ferric ion concentration, hydrogen partial pressure, temperature,rate of reaction). The reaction rate constant, k, may be modeled usingan Arrhenius form:

$\begin{matrix}{k = {A*^{\frac{- E_{a}}{RT}}}} & (10)\end{matrix}$

where A is a pre-exponential factor, R is the universal gas constant, Tis the temperature in kelvin, and E_(a) is the activation energy for thereaction. By plotting the experimental data as a natural log of thereaction rate constant, k, against the inverse reaction temperaturemeasured in kelvin, and by fitting a linear trend line 610 to theplotted data, the reaction rate constant parameters, A and E_(a), may bedetermined from the ln(k) intercept and the slope of a fitted lineartrend line, respectively. Determining the reaction rate constantactivation energy may help to increase the efficiency of a rebalancingmethod and system for rebalancing electrolytes in a redox flow batterysystem. For example, after determining the reaction rate parameters forthe hydrogen reduction of an electrolyte metal ion (e.g., ferric ion inan IFB), an efficiency of an electrolyte rebalancing method and systemmay be increased.

Turning now to FIG. 7, it illustrates an example redox flow batterysystem 700 comprising a redox flow battery cell. FIG. 7 includes some ofthe same elements as the redox flow battery system shown in FIG. 0.Elements in FIG. 7 that are the same as elements in FIG. 0 are labeledwith the same numeric identifiers. For the sake of brevity, adescription of same elements between FIG. 0 and FIG. 7 may be omitted;however, the description of elements in FIG. 0 applies to the elementsin FIG. 7 that have the same numerical identifiers. As shown in FIG. 7,positive electrolyte source 52 and negative electrolyte source 50 mayeach hold liquid electrolyte comprising positive electrolyte 756 andnegative electrolyte 754, respectively. As shown in FIG. 7, positiveelectrolyte 756 may recirculate through the positive electrodecompartment 22, and negative electrolyte 754 may recirculate to thenegative electrode compartment 20. In the redox flow battery system 700,the positive electrolyte source 52 and the negative electrolyte source50 may both purged with inert gas such as Ar to remove oxygen gas. Thepurged electrolytes may be pumped via pumps 32 and 30 through thepositive and negative sides of the battery, respectively. The positiveand negative sides of the battery may refer to the positive electrodecompartment 22 and the negative electrode compartment 20. Two tricklebed reactors 710, 712 comprising catalyst beds 740, 742 respectively,may be connected in-line with the recirculating flow paths of theelectrolyte at the negative and positive sides of the battery,respectively, in the redox flow battery system 700. In one example, thetrickle bed reactors 710, 712 may be placed in the flow path of thepositive and negative electrolyte sources 50 and 52.

During battery charge, gaseous hydrogen may be generated on the negativeside of the battery (e.g., at negative electrode 26) due to bothelectrochemical and corrosion side reactions (equations (3), and (4)) aspreviously described. Equation (4) is written for corrosion of ironmetal electrode, for example in an IFB system, however, corrosion ofother metals producing hydrogen gas may also occur in other types ofhybrid redox flow battery systems or other redox flow battery systems.The hydrogen generated from the electrochemical and corrosion sidereactions may accumulate at the negative electrolyte source 50 and thepositive electrolyte source 52. A pressure equalization line 704 mayconnect negative source 50 and positive electrolyte source 52, therebyequating a pressure between a gas head spaces 757 and 755 of positiveand negative electrolyte sources, respectively. In this manner, hydrogengas may be distributed to the recirculating flow paths of theelectrolyte at the negative and positive sides of the battery,respectively. In particular, the hydrogen gas may be supplied to thetrickle bed reactors 710, 712. Ejectors 730 and 732 may be locatedbetween the outlet of the negative electrode compartment 20 and thepositive electrode compartment 22 of battery cell 18 and trickle bedreactors 710 and 712, respectively. Ejectors 730 and 732 may deliver apredetermined amount or flow rate of hydrogen gas to trickle bedreactors 710 and 712, respectively. Ejectors 730, 732 may be connectedto the gas head space 757 of positive source 52 and the gas head space755 of negative electrolyte source 50. For example, negative electrolyteflowing from the negative electrode compartment 20 may pass throughejector 730, thereby drawing gas (e.g., hydrogen gas) from gas headspace 757 of positive electrolyte source 52, and positive electrolyteflowing from the positive electrode compartment 22 may pass throughejector 732, thereby drawing gas (e.g., hydrogen gas) from gas headspace 755 of negative electrolyte source 50. The sizes of ejectors 730and 732 may be predetermined based on a predetermined amount of hydrogengenerated and a predetermined speed of the reduction reaction. Forexample, the sizes of ejectors 730 and 732 may be increased to increasethe hydrogen gas flow to the trickle bed reactors 710, 712,respectively, relative to the flow of electrolyte recirculated by pumps30, and 32, respectively. In some examples, the sizes of ejectors 730,and 732 may be different, the sizes of each ejector predeterminedaccording to the predetermined hydrogen flow rates to trickle bedreactors 710 and 712. For example, in an IFB, because the ferric ionconcentration may be higher in the positive electrolyte at the positiveside of the redox flow battery system, a larger portion of the hydrogengas may be drawn through ejector 732. As a further example, the ejectorsmay be sized according to the reaction rate parameters determined asdiscussed above for equation (9) and system conditions such as reactiontemperature. As a further example, ejectors 730 and 732 may alsocomprise mechanical pumps for delivering liquid electrolyte and hydrogengas to trickle bed reactors 710 and 712, respectively, wherein themechanical pumps may be controlled by controller 80.

Because the amount of generated hydrogen in the redox flow batterysystem may be approximately equal to the amount of unbalanced ferricions, recirculating the generated hydrogen to both the positiveelectrolyte source 52 (and positive electrode compartment 22) and thenegative electrolyte source 50 (and negative electrode compartment 20),may aid in completely rebalancing the electrolytes. For example,recirculating the generated hydrogen to the negative electrolyte source50 may aid in rebalancing free ferric ions that crossover throughseparator 24 from the positive electrode compartment 22.

Redox flow battery system 700 may further comprise an external source790 of hydrogen gas. External source 790 may supply additional hydrogengas to the positive electrolyte source 52 and the negative electrolytesource 50. External source 790 may alternately supply additionalhydrogen gas to the inlet of trickle bed reactors 710, 712. As anexample, a mass flow meter or other flow controlling device (which maybe controlled by controller 80) may regulate the flow of the hydrogengas from external source 790. The external source of hydrogen maysupplement the hydrogen gas generated in redox flow battery system 700.For example, when gas leaks are detected in redox flow battery system700 or when the reduction reaction rate is too low at low hydrogenpartial pressure, an external source of hydrogen gas may be supplied inorder to rebalance the state of charge of the electro-active species inthe positive electrolyte and negative electrolyte. As an example,controller 80 may supply hydrogen gas from external source 790 inresponse to a measured change in pH or in response to a measured changein state of charge of an electrolyte or an electro-active species. Forexample an increase in pH of the negative electrolyte source 50, or thenegative electrode compartment 20, may indicate that hydrogen is leakingfrom the redox flow battery system 700 and/or that the reaction rate istoo slow with the available hydrogen partial pressure, and controller80, in response to the pH increase, may increase a supply of hydrogengas from external source 790 to the redox flow battery system 700. As afurther example, controller 80 may supply hydrogen gas from externalsource 790 in response to a pH change, wherein the pH increases beyond afirst threshold pH or decreases beyond second threshold pH. For example,a first threshold pH for the negative electrolyte may be 4 and a secondthreshold pH for the negative electrolyte may be 3. In other words ifthe pH for the negative electrolyte is measured beyond a first range(e.g., less than 3 or greater than 4), then controller 80 may adjust(e.g., increase or decrease, or shut off, etc.) the external hydrogengas supply rate to return the pH to the first range. As another example,if the pH of the negative electrolyte is greater than 4, then controller80 may increase the external gas supply rate to supply additionalhydrogen to increase the rate of reduction of ferric ions and the rateof production of protons, thereby reducing the positive electrolyte pH.Furthermore, the negative electrolyte pH may be lowered by hydrogenreduction of ferric ions crossing over from the positive electrolyte tothe negative electrolyte or by proton generated at the positive sidecrossing over to the negative electrolyte due to a proton concentrationgradient and electrophoretic forces. In this manner, the pH of thenegative electrolyte may be maintained within the stable region from3-4, while reducing the risk of precipitation of ferric ions (crossingover from the positive electrode compartment) to Fe(OH)₃. Other controlschemes for controlling the supply rate of hydrogen gas from externalsource 790 responsive to a change in an electrolyte pH or to a change inan electrolyte state of charge, detected by other sensors such as anoxygen-reduction potential (ORP) meter or an optical sensor, may beimplemented. Further still, the change in pH or state of chargetriggering the action of controller 80 may be based on a rate of changeor a change measured over a time period. The time period for the rate ofchange may be predetermined or adjusted based on the time constants forthe redox flow battery system. For example the time period may bereduced if the recirculation rate is high, and local changes inconcentration (e.g., due to side reactions or gas leaks) may quickly bemeasured since the time constants may be small.

Turning now to FIG. 8, it shows a cross section of an example cell 800of a redox flow battery system. As an example, a hybrid all-iron flowbattery system is described with reference to FIG. 8, however examplecell 800 may be representative for any redox flow battery system. Cell800 may comprise a redox plate 812, comprising, for example, carbon orgraphite. Cell 800 may further comprise a positive electrode 814 thatmay be positioned directly adjacent to the redox plate 812, and anegative electrode 818. As an example, the positive electrode 814 maycomprise graphite. For the case of an IFB, negative electrode 818 maycomprise a plating iron electrode. A separator 816, for example anion-exchange membrane, may be positioned between to the positiveelectrode and the negative electrode, one surface of the separator 816being immediately adjacent to the positive electrode, and anothersurface of the separator 816 being immediately adjacent to the negativeelectrode 818. In the cell 800, positive electrode 812 may comprise apositive electrode compartment containing a positive electrolytetherein, and a negative electrode 818 may comprise a negative electrodecompartment containing a negative electrolyte therein. Furthermore,separator 816 may be an electrically insulating ion conducting barrierthat separates a positive electrolyte recirculating through the positiveelectrode 814 and a negative electrolyte recirculating through thenegative electrode 818. The positive electrolyte and the negativeelectrolyte may recirculate to the cell 800 from a positive electrolytesource (not shown) and a negative electrolyte source (not shown),respectively, via pumps (not shown), similar to the configuration ofFIG. 7. For example, positive electrolyte and negative electrolyte mayrecirculate to and from the positive electrode 814 and negativeelectrode 818 in a direction perpendicular to the cross-section surfaceof FIG. 8. External sources (not shown) for supplying hydrogen, acids,additives, or combinations thereof to the cell 800 may also be provided.

For the case of a redox flow battery system comprising a plurality ofcells stacked in series or in parallel, a redox plate of an adjacentredox flow battery cell may be positioned immediately adjacent to a backface of the negative electrode, wherein a back face of the negativeelectrode is opposite to the face of the negative electrode contactingseparator 816.

In one example, cell 800 may comprise a cell for a hybrid redox flowbattery system, for example, an IFB system. In an IFB system, thenegative electrode 818 may include a substrate structure on which theiron metal, Fe⁰, may plate (solidify) during IFB charge, the positiveelectrolyte may comprise ferric ion, and ferrous ion, and the negativeelectrolyte may comprise ferrous ion. Accordingly, the positiveelectrolyte may comprise a first metal ion and the negative electrolytemay comprise a second metal ion, wherein an oxidation state of the firstmetal ion may be higher than an oxidation state of the second metal ion.

FIGS. 9-10 include some of the same elements as the redox flow batterycell described shown in FIG. 8. Elements in FIGS. 9-10 that are the sameas elements in FIG. 8 are labeled with the same numeric identifiers. Forthe sake of brevity, a description of same elements between FIG. 8 andFIGS. 9-10 may be omitted; however, the description of elements in FIG.8 applies to the elements in FIGS. 9-10 that have the same numericalidentifiers.

Turning now to FIG. 9, it illustrates a cross-section of an exampleredox flow battery cell 900 comprising a catalyst layer 910. Catalystlayer 910 may comprise a thin layer of catalyst that may catalyze thehydrogen reduction reaction (e.g., equation (6)). As previouslydiscussed, the catalyst layer may comprise a graphite, orcarbon-supported precious metal. The thin layer of catalyst may beapplied to a surface of the separator 816 facing towards the negativeelectrode 818 using a method comprising spraying, doctor blade coating,screen printing, hot-pressing, and the like. For the case where thecatalyst and the catalyst support are electrically conductive, aninsulating barrier 920, for example, a micro-porous membrane, may beapplied between the catalyst layer 910 and the negative electrode 818 toinsulate the negative electrode 818 from the catalyst layer 910.Insulating barrier 920 may be electrically insulative, but may bepermeable to fluids, including hydrogen gas generated at the negativeelectrode and the electrolyte.

Accordingly, in the redox flow battery cell 900, hydrogen generated atthe negative electrode 818 may react with electro-active species such asa metal ion (e.g., ferric ion in an IFB system) at the interface betweenthe separator 816 and the negative electrode 818. For example, ferricions crossing over the separator 816 from the positive electrode 814 maybe reduced by the hydrogen gas produced at the negative electrode. Inthis way, the hydrogen reduction reaction may occur at/or near theposition where the hydrogen gas is generated so that the hydrogen gasmay be quickly oxidized (e.g., when reducing ferrous ion to ferric ion)to produce protons. Thus, the redox flow battery cell 900 may moreefficiently maintain a substantially balanced electrolyte state ofcharge and also more efficiently maintain a substantially stableelectrolyte pH as compared to other redox flow battery cellconfigurations where the hydrogen reduction reaction for rebalancing theelectrolytes takes place at a location further away from the negativeelectrode. For example, the redox flow battery system of FIG. 9comprising catalyst layer 910 may achieve improved metal ion reductionreaction efficiency as compared to redox flow battery systems employingauxiliary rebalancing cells. As another example, the redox flow batterysystem of FIG. 9 comprising catalyst layer 910 may achieve improvedmetal ion reduction reaction efficiency as compared to redox flowbattery systems employing external catalyst beds such as trickle bedreactors 740 and 742. In other examples a redox flow battery may employa combination of an external catalyst bed such as trickle bed reactor742, and an internal catalyst surface such as catalyst layer 910 toimprove metal ion reduction reaction efficiency.

Turning now to FIG. 10, it illustrates a schematic of an example redoxflow battery cell 1000 comprising a non-conductive supported catalystlayer 1010 positioned immediately adjacent to the separator 816 on thesurface of the separator 816 facing the negative electrode 818.Non-conductive supported catalyst layer 1010 may comprise, for example,Al₂O₃, ceramic materials, and the like. Coating methods comprisingspraying, doctor blade coating, screen printing, hot-pressing, and thelike, or any combination thereof may be used to apply the non-conductivesupported catalyst layer 1010 to the separator 816. Supporting the thincatalyst layer on a non-conductive support may simplify the redox flowbattery cell 1000 as compared to the redox flow battery cell 900 becauseredox flow battery cell 1000 may not comprise an insulating barrier 920between the catalyst layer and the negative electrode.

In this manner, a redox flow battery system may comprise a positiveelectrode in fluid communication with a positive electrolyte comprisinga first metal ion, a negative electrode in fluid communication with anegative electrolyte comprising a second metal ion, an electricallyinsulating ion conducting surface separating the positive electrode fromthe negative electrode, and a catalyst surface in fluid communicationwith the first metal ion, the second metal ion, or a combinationthereof, and hydrogen gas, wherein the hydrogen gas and the first metalion, the second metal ion, or a combination thereof are fluidlycontacted at the catalyst surface. The first metal ion may comprise ametal ion having a higher oxidation state, and the second metal ion maycomprise a metal ion having a lower oxidation state. In one example, theredox flow battery system may comprise an iron redox flow batterysystem, the negative electrode may comprise an iron plating electrode,and the positive electrolyte and the negative electrolyte may comprise aferric ion, ferrous ion, or a combination thereof. The redox flowbattery system may further comprise an external source of hydrogen gasin fluid communication with the catalyst surface, and the catalyst maycomprise a catalyst bed in a trickle bed reactor.

The redox flow battery system may further comprise a pump fluidlypositioned between the positive electrode and the trickle bed reactor,wherein the pump supplies the hydrogen gas and the positive electrolyteto the trickle bed reactor. The pump may comprise an ejector, wherein aflow of the positive electrolyte through the ejector draws the hydrogengas through the ejector to the trickle bed reactor, and the catalyst maycomprise a catalyst layer positioned between the electrically insulatingion conducting surface and the negative electrode. The redox flowbattery system may further comprise an electrically-insulativefluid-permeable substrate between the catalyst layer and the negativeelectrode, wherein the catalyst layer is disposed on theelectrically-insulative fluid-permeable substrate. The catalyst layermay comprise one or more of a graphite catalyst layer and a preciousmetal-based catalyst layer.

Turning now to FIG. 11, it illustrates flowchart for an example method1100 of rebalancing electrolytes in a redox flow battery system. Method1100 begins at 1110, where redox flow battery system operatingconditions may be determined. As an example, at 1110, electrolytechemical properties including pH, battery state of charge, electrolyteconcentrations, electrolyte state of charge, and the like may bemeasured using various sensors and/or probes (e.g., sensors 60, 62, 70,72). As an example, the battery state of charge may be determined usingan optical sensor, and the pH may be measured using a pH meter, andelectrolyte concentration may be monitored using an ORP probe formeasuring electrolyte potential. Next, method 1100 continues at 1120where hydrogen gas may be directed from positive and/or negativeelectrodes to a catalyst surface. As an example, hydrogen that may begenerated at a negative electrode from corrosion and/or electrochemicalside reactions (e.g., equations (3)-(6)) may be directed to a catalystsurface via pumps and/or ejectors (e.g., ejectors 730, 732; pumps 30,32). As a further example, hydrogen from an external source (e.g.external source 790) may be directed to a catalyst surface. The catalystsurface may comprise catalyst surfaces of a packed catalyst bed (e.g.,740 or 742) and/or a catalyst layer (e.g., 910 or 1010) positionedbetween the separator and the negative electrode. For the case of acatalyst layer, e.g., catalyst layer 910 or non-conductive supportedcatalyst layer 1010 is positioned relative to the negative electrodeand/or positive electrode so that the hydrogen may be directed, or maybe self-directed (e.g., hydrogen gas may rise) to the catalyst surface.Other catalyst surfaces that catalyze the hydrogen reduction reaction(e.g., equation (6)) may be employed. For example, the catalyst surfacecomprising a packed catalyst bed may include a packed catalyst bedwithin a reactor type other than a trickle bed reactor.

Next, method 1100 continues at 1130, where electrolyte may be directedto the catalyst surface. The electrolyte may comprise a positiveelectrolyte and/or a negative electrolyte, and the electrolyte maycomprise one or multiple metal ions. The electrolyte may be directed toa catalyst surface via pumps and/or ejectors (e.g., ejectors 730, 732;pumps 30, 32). As a further example, electrolytes may be directed to acatalyst surface from an external source. As a further example, thecatalyst surface may be positioned relative to the negative electrodeand/or positive electrode so that the electrolyte may be directed, ormay be self-directed (e.g., electrolyte may gravity flow) to thecatalyst surface.

At 1140, method 1100 comprises fluidly contacting the hydrogen gas withthe electrolyte at the catalyst surface. As an example, fluidlycontacting the hydrogen gas with the electrolyte at the catalyst surfacemay comprise mixing the hydrogen gas with a liquid electrolyte andinjecting the gas-liquid mixture into a trickle bed reactor, therebyfluidly contacting the gas-liquid mixture at a catalyst surfacecomprising a packed catalyst bed of a trickle bed reactor. As a furtherexample, fluidly contacting the hydrogen gas with the electrolyte at thecatalyst surface may comprise positioning a catalyst surface relative toa positive and/or negative electrode wherein hydrogen gas andelectrolyte are each directed to and fluidly contact each other at thecatalyst surface (e.g., fluidly contacting the hydrogen gas withelectrolyte at the thin catalyst layer 910 or 1010).

Next, at 1150, method 1100 comprises determining if a change in pH isdetected, for example, to control a negative electrolyte pH within apre-determined range. Determining a pH change may comprise measuring achange in pH with an appropriate pH meter or other sensor and/or probepositioned, for example, at a battery cell or electrolyte source.Detecting a change in pH may comprise detecting a pH increase or a pHdecrease. Furthermore, detecting a change in pH may comprise detecting apH increase or pH decrease beyond a first pH range or a second pH range.For example, in an IFB, the first pH range may be from 3 to 4,corresponding to a pH range over which the negative electrolyte isstable, and the second pH range may be from 1 to 2, corresponding to apH range over which the positive electrolyte is stable. As anotherexample, the first pH range may correspond to a pH less than 4. Thefirst pH range and the second pH range may be predetermined depending onthe particular redox flow battery system. For example, a Pourbaixdiagram may be used to predetermine the first pH range and the second pHrange for the redox flow battery system. If the detected pH is withinthe predetermined range, method 1100 continues at 1160, where it isdetermined if an electrolyte state of charge (SOC) imbalance isdetected. Detecting an electrolyte SOC imbalance may comprise measuringa change in one or a plurality of electrolyte concentrations, measuringa change in one or a plurality of electrolyte SOC's, and the like. Forexample, if the total concentration of ferric ions in the positiveelectrolyte, indicated by its ORP, is substantially imbalanced with thetotal concentration of ferrous ions, indicated by its ORP, in thenegative electrolyte in an IFB system, then an electrolyte SOC imbalancemay be detected. As a further example, the electrolyte SOC imbalance maybe determined by averaging measured electrolyte concentrations orelectrolyte SOCs over a predetermined time interval. For example, duringa predetermined time interval at a known charging or dischargingcurrent, the exact amount of electrons transferred in coulombs can becalculated by multiplying the charging or discharging current in Amperesby the predetermined time interval in seconds. Based on the totalcoulombs transferred and the positive and negative reactions for a redoxflow battery system, changes in the amounts of species in the positiveand negative electrolytes can be determined. If the measured electrolyteSOC does not balance with changes in the amounts of species calculatedfrom the total coulombs transferred, then an electrolyte SOC imbalancemay be determined. If an electrolyte SOC imbalance is not detected,method 1100 ends.

If a negative electrolyte pH is above the predetermined pH range at1150, or an electrolyte SOC imbalance is detected at 1160, method 1100continues at 1170, where external hydrogen may be supplied from anexternal source. For example, in an IFB, if the pH of the negativeelectrolyte increases beyond a first range corresponding to the rangewhere the ferric ions is stable, hydrogen gas may be supplied (e.g., viaa controller) to the IFB cell to drive the reduction of ferric ion atthe catalyst surface. In this way, the supplied hydrogen gas from anexternal source may increase the hydrogen partial pressure at thecatalyst surface, and may thereby speed up the hydrogen reduction offerric ion at the catalyst surface, producing protons and reducing thepH of the positive electrolyte. A controller may also supply externalhydrogen responsive to a detected electrolyte SOC imbalance. Suppliedhydrogen gas from an external source may increase the rate of hydrogenreduction of ferric ion at the catalyst surface, thereby rebalancing theelectrolyte SOC in the positive and negative electrolytes.

In this manner, a method of rebalancing electrolytes in a redox flowbattery system may comprise directing hydrogen gas in the redox flowbattery system to a catalyst surface, fluidly contacting the hydrogengas with an electrolyte comprising a metal ion at the catalyst surface;and chemically reducing the metal ion by the hydrogen gas at thecatalyst surface, wherein a state of charge of the electrolyte remainssubstantially balanced. In another example, a method of rebalancingelectrolytes in a redox flow battery system may comprise directinghydrogen gas in the redox flow battery system to a catalyst surface,fluidly contacting the hydrogen gas with an electrolyte comprising ametal ion at the catalyst surface, and chemically reducing the metal ionby the hydrogen gas at the catalyst surface, wherein a pH of theelectrolyte remains within a predetermined range. In a further example,a method of rebalancing electrolytes in a redox flow battery system maycomprise directing hydrogen gas in the redox flow battery system to acatalyst surface, fluidly contacting the hydrogen gas with anelectrolyte comprising a metal ion at the catalyst surface, andchemically reducing the metal ion by the hydrogen gas at the catalystsurface, wherein a state of charge of the electrolyte remainssubstantially balanced and a pH of the electrolyte remains within apredetermined range.

The methods of rebalancing electrolytes in a redox flow battery systemmay further comprise measuring a pH of the electrolyte and supplyinghydrogen from an external source in response to a change in the pH ofthe electrolyte. The methods may further comprise measuring a state ofcharge of the electrolyte and supplying hydrogen from an external sourcein response to a change in the state of charge of the electrolyte.Further still, the redox flow battery system may comprise an iron redoxflow battery system, and the metal ion may comprise ferric ion.

Directing the hydrogen gas to the catalyst surface may comprisedirecting the hydrogen gas to a catalyst bed in a trickle bed reactor,wherein fluidly contacting the hydrogen gas with the electrolyte at thecatalyst surface comprises fluidly contacting the hydrogen gas with theelectrolyte at the catalyst bed of the trickle bed reactor. Furthermore,the redox flow battery system may comprise a positive electrode and anegative electrode, wherein directing the hydrogen gas to the catalystsurface may comprise directing the hydrogen gas to a catalyst layerpositioned at an electrically insulating ion conducting surfaceseparating the positive electrode and the negative electrode.

In this manner, a method of operating a redox flow battery system maycomprise recirculating a positive electrolyte to a positive electrode,recirculating a negative electrolyte to a negative electrode, directinghydrogen gas to a first catalyst surface, directing the positiveelectrolyte to the first catalyst surface, and fluidly contacting thehydrogen gas with the positive electrolyte at the first catalystsurface, wherein the positive electrolyte is chemically reduced by thehydrogen gas at the first catalyst surface, a positive electrolyte pH ismaintained within a first range, and a state of charge of the positiveelectrolyte and a state of charge of the negative electrolyte remainsubstantially constant. The method may further comprise measuring thepositive electrolyte pH, measuring the negative electrolyte pH, andsupplying hydrogen gas from an external source to the redox flow batterysystem in response to at least one of a change in the positiveelectrolyte pH and the negative electrolyte pH. Directing hydrogen gasto a first catalyst surface may comprise directing the hydrogen gas to afirst catalyst bed in a first trickle bed reactor, and wherein fluidlycontacting the hydrogen gas with the positive electrolyte at the firstcatalyst surface comprises fluidly contacting the hydrogen gas with thepositive electrolyte at the first catalyst bed in the first trickle bedreactor.

The method of operating the redox flow battery system may furthercomprise directing hydrogen gas to a second catalyst bed in a secondtrickle bed reactor, directing the negative electrolyte to the secondcatalyst bed, and fluidly contacting the hydrogen gas with the negativeelectrolyte at the second catalyst bed, wherein the negative electrolyteis chemically reduced by the hydrogen gas at the second catalystsurface, and a negative electrolyte pH is maintained within a secondrange. Directing hydrogen gas to a first catalyst surface may comprisedirecting the hydrogen gas to a catalyst layer adjacent to anelectrically insulating ion conducting surface separating the positiveelectrode from the negative electrode.

As will be appreciated by one of ordinary skill in the art, the methodsdescribed in FIG. 11 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

It will also be appreciated that the configurations and routinesdisclosed herein are exemplary in nature, and that these specificembodiments are not to be considered in a limiting sense, becausenumerous variations are possible. For example, the above technology maybe applied to other flow battery types. The subject matter of thepresent disclosure includes all novel and nonobvious combinations andsubcombinations of the various systems and configurations, and otherfeatures, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application.

Such claims, whether broader, narrower, equal, or different in scope tothe original claims, also are regarded as included within the subjectmatter of the present disclosure.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,hybrid redox flow battery systems, all-iron hybrid redox flow batterysystems, and other redox flow battery systems may all take advantage ofthe present description.

1. A method of rebalancing electrolytes in a redox flow battery system, comprising: directing hydrogen gas in the redox flow battery system to a catalyst surface; fluidly contacting the hydrogen gas with an electrolyte comprising a metal ion at the catalyst surface; and chemically reducing the metal ion by the hydrogen gas at the catalyst surface, while a state of charge of the electrolyte remains substantially balanced, or a pH of the electrolyte remains within a predetermined range.
 2. The method of claim 1, further comprising measuring a pH of the electrolyte and supplying hydrogen from an external source in response to a change in the pH of the electrolyte.
 3. The method of claim 1, further comprising measuring a state of charge of the electrolyte and supplying hydrogen from an external source in response to a change in the state of charge of the electrolyte.
 4. The method of claim 1, wherein the redox flow battery system comprises an iron redox flow battery system, and the metal ion comprises ferric ion, ferric complexes, or a combination thereof.
 5. The method of claim 1, wherein the redox flow battery system comprises a positive electrode and a negative electrode, and wherein directing the hydrogen gas to the catalyst surface comprises directing the hydrogen gas to a catalyst bed in a trickle bed reactor placed in the flow path of positive and negative electrolyte, and wherein fluidly contacting the hydrogen gas with the electrolyte at the catalyst surface comprises fluidly contacting the hydrogen gas with the electrolyte at the catalyst bed of the trickle bed reactor.
 6. The method of claim 1, wherein the redox flow battery system comprises a positive electrode and a negative electrode, and wherein directing the hydrogen gas to the catalyst surface comprises directing the hydrogen gas to a catalyst layer positioned at an electrically insulating ion conducting surface separating the positive electrode and the negative electrode.
 7. A redox flow battery system, comprising: a positive electrode in fluid communication with a positive electrolyte comprising a first metal ion; a negative electrode in fluid communication with a negative electrolyte comprising a second metal ion; an electrically insulating ion conducting surface separating the positive electrode from the negative electrode; and a catalyst surface in fluid communication with the first metal ion, the second metal ion, or a combination thereof, and hydrogen gas, wherein the hydrogen gas and the first metal ion, the second metal ion, or a combination thereof are fluidly contacted at the catalyst surface.
 8. The redox flow battery system of claim 7, wherein the first metal ion comprises a metal ion having a higher oxidation state, and wherein the second metal ion comprises a metal ion having a lower oxidation state.
 9. The redox flow battery system of claim 7, wherein the redox flow battery system comprises an iron redox flow battery system, the negative electrode comprises an iron plating electrode, and the positive electrolyte and the negative electrolyte comprise ferric ion, ferrous ion, or a combination thereof.
 10. The redox flow battery system of claim 7, further comprising an external source of hydrogen gas in fluid communication with the catalyst surface.
 11. The redox flow battery system of claim 7, wherein the catalyst comprises a catalyst bed in a trickle bed reactor.
 12. The redox flow battery system of claim 11, further comprising a pump fluidly positioned between the positive electrode and the trickle bed reactor, wherein the pump supplies the hydrogen gas and the positive electrolyte to the trickle bed reactor.
 13. The redox battery system of claim 12, wherein the pump comprises an ejector, wherein a flow of the positive electrolyte through the ejector draws the hydrogen gas through the ejector to the trickle bed reactor.
 14. The redox flow battery system of claim 7, wherein: the catalyst comprises a catalyst layer positioned between the electrically insulating ion conducting surface and the negative electrode.
 15. The redox flow battery system of claim 14, further comprising an electrically-insulative fluid-permeable substrate between the catalyst layer and the negative electrode, wherein the catalyst layer is disposed on the electrically-insulative fluid-permeable substrate.
 16. The redox flow battery system of claim 14 wherein the catalyst layer comprises one or more of a graphite catalyst layer and a precious metal-based catalyst layer.
 17. A method of operating a redox flow battery system, comprising: recirculating a positive electrolyte to a positive electrode; recirculating a negative electrolyte to a negative electrode; directing hydrogen gas to a first catalyst surface; directing the positive electrolyte to the first catalyst surface; and fluidly contacting the hydrogen gas with the positive electrolyte at the first catalyst surface, wherein the positive electrolyte is chemically reduced by the hydrogen gas at the first catalyst surface; a positive electrolyte pH is maintained within a first range; and a state of charge of the positive electrolyte and a state of charge of the negative electrolyte remain substantially constant.
 18. The method of operating the redox flow battery system of claim 17, further comprising; measuring the positive electrolyte pH; measuring the negative electrolyte pH; and supplying hydrogen gas from an external source to the redox flow battery system in response to at least one of a change in the positive electrolyte pH and the negative electrolyte pH.
 19. The method of operating the redox flow battery system of claim 17, wherein directing hydrogen gas to a first catalyst surface comprises directing the hydrogen gas to a first catalyst bed in a first trickle bed reactor, and wherein fluidly contacting the hydrogen gas with the positive electrolyte at the first catalyst surface comprises fluidly contacting the hydrogen gas with the positive electrolyte at the first catalyst bed in the first trickle bed reactor.
 20. The method of operating the redox flow battery system of claim 18, further comprising: directing hydrogen gas to a second catalyst bed in a second trickle bed reactor; directing the negative electrolyte to the second catalyst bed; and fluidly contacting the hydrogen gas with the negative electrolyte at the second catalyst bed; wherein the negative electrolyte is chemically reduced by the hydrogen gas at the second catalyst surface, and a negative electrolyte pH is maintained within a second range. 